Controller for precisely powering remote radio heads on a cell tower

ABSTRACT

Disclosed is a system and method for providing stable and reliable power to components on the top of a cell tower. The system performs a device discovery process to determine with Power Supply Units are connected to which Remote Radio Heads on the tower. It also provides several ways of characterizing the power cables and input capacitance to the Remote Radio Heads to provide optimal power to the Remote Radio Heads, including situations in which the power demand for the Remote Radio Heads increases, while obviating the need to replace the power cables with those of greater current capacity. Further, the system provides for stable power even in the presence of sensor instabilities and data dropouts.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a system and method for providing precise and robustpower to a plurality of Remote Radio Heads (RRHs) mounted on a celltower.

Background

The increasing sophistication and capabilities of Remote Radio Heads(RRHs) mounted on cell towers has led to greater power demands. In atypical tower installation, each RRH includes several components (e.g.,antenna, a plurality of diplexers and filters, and a Remote ElectricalTilt (RET) mechanism), each of which corresponding to a sector. Thepower supplies for each RRH is located either in a shelter at the baseof the tower, or at a location some distance from the base of the tower.

FIG. 1 illustrates a conventional cell tower deployment 100, whichincludes a tower 110 on which are mounted three Remote Radio Heads 120(RRH), including RRH1, RRH2, and RRH3. These RRHs 120 receive power fromrespective Power Supply Units 130 (PSU), which includes PSU1, PSU2, andPSU3. Each PSU provides power to its respective RRH 110 via power cable140. Power cable 140 may be a hybrid power cable that includes a signalline (not shown) that carries information to/from RRH 120. Each of thePSUs 130 may be housed in a shelter 150, which may be located at thebase of tower 110, or may be located a distance from tower 110, wherebythe maximum distance is limited by the power losses in hybrid cable 140.

One problem with increasing power demand is that simply increasing thenumber of power cables 140, or replacing existing power cables 140 withthose having increased capacity, can be prohibitively expensive. Asolution to this is to increase the capacity of existing power cablingby increasing the voltage at the PSUs 130 and keeping the currentrelatively constant. This requires precise and stable control of thevoltage outputs of the PSUs 130.

Further, conventional solutions to providing power to RRHs 120 involvehaving a technician set physical addresses on the components, typicallywith hardware switches integrated into the components. If the techniciansets the wrong address, the PSUs 130 may be mismatched to theirrespective RRHs, leading to improper powering and requiring that atechnician return to the top of the tower 110 to correct the addressanomaly, which can also be prohibitively expensive.

Finally, there is typically a sensor at the tower-end of the power cable140, which senses the received voltage, digitizes it, and transmits thesensed voltage back to the respective PSU 130. These sensors, and theirrespective digitizers, may be affected by extreme environments, both interms of weather as well as electromagnetic environments. This mayaffect the stability and performance of the digitizers, whereby thedevice may transmit spurious data and noisy data to the PSUs 130. Thisin turn degrades the precision of the voltage that the PSU 130 canprovide to the RRHs 120 on the tower.

Accordingly, there is a need for a power supply controller for celltowers, whereby the power supply controller can make optimal use ofexisting cable infrastructure, can detect anomalies in RRH power readingand configuration settings, and can provide robust and precise power toeach RRH in the presence of challenging physical environments.

SUMMARY OF THE INVENTION

An aspect of the present invention is a system for controlling power toa cell tower. The system comprises a plurality of remote radio heads,each of the remote radio heads having a corresponding voltage sensor; aplurality of power supply units; and a controller coupled to the voltagesensors and the plurality of power supply units, wherein the controlleris configured to receive a plurality of voltage samples from each of theplurality of voltage sensors, the voltage samples corresponding to powerprovided by each of the power supply units to a corresponding remoteradio head, pre-filter each voltage sample, and issue a voltage commandto each of the power supply units, wherein each voltage commandcorresponds to a pre-filtered voltage sample

Another aspect of the present invention is a method for initializing apower system for a cell tower, the power system having a plurality ofpower supply units coupled to a respective plurality of remote radioheads by a plurality of power cables. The method comprises: a) setting afirst power supply unit to a first frequency; b) sending an output pulseat the first frequency; c) measuring a return time for a return pulsecorresponding to the output pulse; d) measuring an amplitude of thereturn pulse; e) repeating steps a)-d) for a plurality of frequencies;f) identifying a matching frequency corresponding to a highest amplitudereturn pulse; g) calculating a power cable length based on the returntime corresponding to the matching frequency; h) calculating a powercable resistance based on the power cable length; and i) calculating anoffset voltage based on the power cable resistance.

Another aspect of the present invention is a power system for a celltower. The power system comprises a plurality of remote radio heads; aplurality of power supply units, each of the power supply units coupledto a respective remote radio head by a powercable; a inductive currentsensor coupled to each of the power supply units and its respectivepower cable; and a controller coupled to the plurality of power supplyunits and its corresponding inductive current sensor, the controllerhaving a processor and a non-volatile machine-readable memory encodedwith instructions for operating in a test mode and an operation mode,wherein the test mode includes setting a series of voltages at each ofthe plurality of power supply units, measuring a resulting current witha corresponding inductive current sensor and calculating a VI factorcorresponding to each voltage and the current, and wherein the operationmode includes setting each of the power supply units to a controlvoltage in response to a sensed current, wherein the control voltage isa function of the sensed current and a respective VI factor.

And another aspect of the present invention is a method for controllinga power system for a cell tower, the power system having a plurality ofpower supply units coupled to a respective plurality of remote radioheads by a plurality of power cables. The method comprises receiving aplurality of voltage samples from a plurality of voltage sensors;pre-filtering the plurality of voltage samples, wherein thepre-filtering includes validating each of the plurality of voltagesamples: comparing each of the pre-filtered voltage samples to a voltagesetpoint; sending a command voltage based on the comparing and storingthe command voltage; and receiving and storing a sensed voltage andsensed current from the power supply unit, wherein the sensed voltageand sensed current correspond to the command voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional RRH tower deployment.

FIG. 2a illustrates an exemplary power control system according to thedisclosure.

FIG. 2b illustrates an exemplary controller according to the disclosure

FIG. 3 illustrates an exemplary process for performing an initialRRH/PSU discovery on startup.

FIG. 4 illustrates an exemplary process for nominal operation of thesystem in which the PSUs are controlled to provide for desired voltagesat the RRHs.

FIG. 5 illustrates an exemplary process for monitoring and controllingthe voltages for the PSUs, as well as monitoring for power anomalies andproviding for robust power control in the event of an anomaly.

FIG. 6 illustrates a second exemplary power control system according tothe disclosure.

FIG. 7 illustrates an initial system characterization process that maybe executed by the system of FIG. 6.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 2a illustrates an exemplary system 200 according to the disclosure.System 200 includes a plurality of tower top equipment, which mayinclude Remote Radio Heads (RRHs) 120 and respective Power Supply Units(PSUs) 130, which may be substantially similar to the RRHs 120 and PSUs130, respectively illustrated in FIG. 1. System 200 further includes acontroller 220; a plurality of voltage sensors 210, each of whichcorresponds to a RRH 120; a sensor data bus 230 that provides dataconnectivity between voltage sensors 210 and controller 220; a PSU databus 235 that provides data connectivity between controller 220 and PSUs130; and power cables 240 that transmit power from PSUs 130 to theirrespective RRH 120.

Although FIG. 2 illustrates Remote Radio Heads 120 as an example oftower top equipment, other forms of equipment that has requires aconsistent voltage and that may consume varying current may be tower topequipment according to the disclosure.

Each RRH 120 may include all the components necessary to operate anantenna for a given sector. These components may include an antenna, oneor more diplexers, one or more filters, Tower Mounted Amplifiers (TMA)and a Remote Electrical Tilt (RET) unit. All these components may bepowered by a corresponding PSU 130, with the power being transmitted viapower cable 140. Power distribution may be provided at the RRH 120whereby the power delivered via power cable 140 is broken out anddistributed to the relevant components.

Voltage sensor 210 measures the voltage at the tower-end of power cable140. Voltage sensor 210 may be integrated into the power distributioncircuitry (not shown) within RRH 120. Alternatively, voltage sensor 120may be integrated into a connector at the tower-end of cable 140.Further, voltage sensor 120 may be integrated into power cable 140 atany point along the length of power cable 140, which may involveadditional functionality that is described below.

An alternative to a voltage sensor 210 may be an inductively coupledsensor, which may be an inductive current sensor and/or an inductivevoltage sensor. The inductively coupled sensor(s) may be coupled to agiven power cable 140 at the top of the cable (e.g., at the remote radiohead 120, at the bottom of the cable (e.g., at the power supply 130), orat both locations.

As illustrated in FIG. 2a , controller 220 is coupled to externalnetwork 260 via network connection 250. External network 260 may includethe network infrastructure of one or more mobile network operators,through which the network operator(s) may interact with components ofsystem 200, including issuing commands to and receiving data from one ormore components of system 200.

FIG. 2b illustrates controller 220, which may include the followingcomponents: one or more processors 222 coupled to a non-volatile memory223; a first bus interface 224 for interfacing with data bus 230; asecond bus interface 226 for interfacing with PSU data bus 235; and athird bus interface 228 for connecting to an external network 260 vianetwork connection 250. Non-volatile memory 223 may be encoded withmachine-readable instructions that, when executed by the processor 222,implements the processes described below. The non-volatile memory 223may also be used for storing historical voltage sensor and commandvoltage data, which is also further described below.

Sensor data bus 230 carries data generated by each voltage sensor 210 tocontroller 220. Sensor data bus 230 may involve a daisy chain-styleconnection between controller 220 and each voltage sensor 210 and may beconfigured as a serial bus whereby a single bus line may connectcontroller 220 to all of the voltage sensors 210. Further, depending onwhether data bus 230 is used to carry additional data to/from each RRH120 to the external network 260, sensor data bus 230 may be implementedwith a fiber optic line, or with a robust but slower connection, such asan RS485 protocol.

FIG. 3 illustrates an exemplary process 300 executed by processor 222within controller 220 for scanning system 200 to identify the RRHs 120and their corresponding PSUs 130 to create and store a set of PSU/RRHpairs and store them in non-volatile memory 223.

In step 310, processor 222 executes instructions to receive voltage datafrom each voltage sensor 210. In doing so, processor 222 receivesvoltage data streams from each voltage sensor 210 via sensor data bus230. If voltage sensors 210 transmit packetized data, or data is aspecific protocol format, processor 222 retrieves voltage samples fromeach of the voltage data streams and may buffer the voltage samples,organized by RRH 120. For example, voltage data from a given voltagesensor 210 may be sent on sensor data bus 230 at a rate of once persecond. Each voltage sample may or may not have a respective time stampreflecting the time at which the voltage sample was acquired by voltagesensor 210. It will be understood that various approaches to this arepossible and within the scope of the disclosure.

At this stage of process 300. PSUs 130 may be in standby or a similardisabled mode whereby their voltage output is unregulated/uncontrolled.

In step 315, processor 222 executes instructions to set a first PSUaddress or identifier. In this example in which multiple PSUs 130 arecoupled to controller 220, controller 222 may have a table of busaddresses, which may be stored in non-volatile memory 223, by which itmay communicate with each individual PSU 130 via second bus interface226. In this case, controller 222 sets an address corresponding to afirst PSU 130.

In step 320, controller 222 issues a command to a first PSU 130 to apredetermined first test voltage and to record the time at which thecommand was issued. The first test voltage may be equal to the maximumvoltage for PSU 130, or it may be a different voltage that would alsodistinguish its output voltage from the standby state of the other PSUs130. Processor 222 may further store the commanded voltage, PSU 130address, and time stamp in non-volatile memory 223.

In step 325, processor 222 receives data from the commanded PSU 130, viaPSU data bus 235, the voltage and current output as measured by PSU 130along with a time stamp. Processor 222 may store these data innon-volatile memory 223.

In step 330, controller 220 receives voltage data from each voltagesensor 210 via sensor data bus 230. This step may be substantiallysimilar to step 310 described above.

In step 335, processor 222 executes instructions to identify which RRH120 corresponds to the commanded PSU 130 by identifying the voltagesensor 210 that is measuring the responsive increase in voltage. Indoing so, processor 222 identifies the voltage sensor 201 (and thus itscorresponding RRH 120) and stores it in non-volatile memory 223 alongwith the commanded voltage and time stamp. Note that one of the voltagesensors 210 would indicate an increase in voltage due to the commandedvoltage setting in step 320, but it may be the case that more than oneother voltage sensors 210 may indicate an increase in voltage for anyreason that might not be connected to the increase in the output of theaddressed PSU. If one or more other voltage sensors 210 are indicating arise in voltage, for whatever reason, processor 222 may identify thesecorresponding RRHs 120 as well and store the relevant information forall of the corresponding RRHs 120.

In step 340, processor 222 executes instructions to set the first PSU130 to a second test voltage. In an exemplary embodiment, the secondtest voltage is the minimum voltage output for PSU 130.

In step 345, controller 220 receives data from the commanded PSU 130,via PSU data bus 235, the voltage and current output as measured by PSU130 along with a time stamp. Processor 222 may store these data innon-volatile memory 223. Step 345 may be substantially similar to step325.

In step 350, controller 220 receives, via data bus 230, voltage datafrom each voltage sensor 210, similarly to steps 310 and 330.

In step 355, processor 222 executes instructions to identify which RRH120 corresponds to the commanded PSU 130 by identifying the voltagesensor 210 that is measuring a responsive decrease in voltage. In doingso, processor 222 identifies the voltage sensor 201 (and thus itscorresponding RRH 120) and stores it in non-volatile memory 223 alongwith the commanded voltage and time stamp. Note that step 355 may besubstantially similar to step 335. Similarly to step 335, it may be thecase that more than one voltage sensor 210 indicates a drop on voltage,only one of which in response to the voltage commanded in step 340. Inthis case, processor 222 will store the data corresponding to eachidentified RRH 120.

In step 360, processor 222 executes instructions to retrieve thevoltages and time stamps stored in steps 330 and 345, and the RRHsidentified in respective executions of steps 335 and 355, fromnon-volatile memory 223. With this data retrieved, processor 222executes instructions to identify which RRH appears to be responding toboth the first test voltage (e.g., maximum voltage) and the second testvoltage (e.g., minimum voltage). And in step 365, processor 222 storesthe resulting of PSU/RRH combinations in non-volatile memory 223.

In step 370, processor 222 retrieves the address corresponding to thenext PSU 230 and sets this address for communication via second businterface 226.

Under process 300, steps 320-370 are repeated for each PSU 130, and thusfor each combination of PSU 130 and corresponding RRH 120, until all ofthe PSUs 130 have been activated, their corresponding RRHs 120identified, and their respective PSU/RRH combinations stored.

As mentioned earlier, voltage sensors 210 may be integrated at any pointwithin power cables 140. In this case, there may be a difference betweenthe voltage measured by voltage sensor 210 and the actual voltageapplied to RRH 120 at the end of power cable 140. In this case,processor 222 may execute instructions to extrapolate and estimate thevoltage at RRH 120 based on the voltage measured by voltage 210, thevoltage and current measured by PSU 130, the known cable length from PSU130 to voltage sensor 210, and the known remaining cable length fromvoltage sensor 210 to RRH 120.

With each PSU/RRH combination established and characterized, system 200may run to continually maintain desired voltages at each RRH 120.

FIG. 4 illustrates an exemplary process 400 by which controller 220maintains desired setpoint voltages at RRHs 120. Controller 220 mayexecute process 400 separately for each PSU/RRH combination or mayexecute each step for all of the PSU/RRH combinations before moving tothe next step. It will be understood that such variations to process 400are possible and within the scope of the disclosure.

In step 410, processor 222 executes instructions to receive voltage datastreams from each voltage sensor 210 via sensor data bus 230. Step 410may be substantially similar to steps 310 and 340 described above.

In step 420, processor 222 executes instructions to pre-filter thevoltage data received in step 410. In doing so, processor 222 retrieveshistorical voltage data and their corresponding timestamps fromnon-volatile memory 223, implements a digital filtering model toidentify past behavior of the voltage signal corresponding to the RRH120 as measured by its respective voltage sensor 210, and identifies anypossible outlier voltage value in the current voltage data. If thepre-filtering implementation identifies the current voltage data as anoutlier, processor 222 adjusts or replaces the voltage with a value thatis more consistent with the historical behavior of the stored voltagedata. It will be understood that many digital filtering algorithms forimplementing pre-filtering are possible and within the scope of thedisclosure.

In step 430, processor 222 executes instructions to compare the voltagesample corresponding to each RRH 120 with a designated setpoint voltage.Note that the voltage sample, at this stage of process 400, might be theadjusted or replacement voltage computed in pre-filtering step 420. Thesetpoint voltage is a value that corresponds to a desired voltage at RRH120. There may be a discrepancy in that the voltage sample (from voltagesensor 210) may be different from the actual voltage at RRH 120, due tothe fact that the voltage sensor 210 may be located at a midpoint alongpower cable 140. In this case step 430 will require a substep in whichprocessor 222 takes the pre-filtered voltage sample and extrapolateswhat the corresponding voltage would be at RRH 120, in other words, ifvoltage sensor 210 is placed at some mid-point along power cable 140,processor 222 may execute instructions to retrieve data fromnon-volatile memory 223 pertaining to the current sensed at the outputof PSU 130, the estimated resistance of power cable 140, and theremaining distance from the voltage sensor 210 and the end of powercable 140 at the corresponding RRH 120. With this data, processor 222may calculate the required voltage offset to compensate for theremaining length of power cable 140. Once processor 222 has calculatedthe voltage offset and added it to the pre-filtered voltage sample,processor 222 may then execute instructions to compare this value to thevoltage setpoint for RRH 120.

In comparing the compensated sensed voltage to the setpoint voltage,processor 222 may execute instructions to adjust the command voltage torestore the sensed voltage to the setpoint voltage. This may be doneaccording to known digital control principles.

Further, given that the length of power cable 140 and its resistance isknown and stored in non-volatile memory 223, processor 222 may executeinstructions to calculate a command voltage offset, which it may add tothe command voltage, to compensate for the estimated voltage drop acrosspower cable 140 between PSH 130 and RRH 120.

Processor 222 may adjust the setpoint voltage for RRH 120 so that thevoltage applied to power cable 140 can be increased, and the currentfrom PSU 130 throttled, so that more power may be delivered to RRH 120with the current remaining controlled. In doing so, PSU 130 may delivermore power to RRH 120 while controlling power dissipation in power cable140, thereby obviating the need to replace power cable 140 with a cableof greater current capacity.

In step 440, processor 222 issues the command voltage to PSU 130 via PSUdata bus 260, and stores the command voltage in non-volatile memory 223.

An additional aspect of the invention is that system controller 220 candetect two types of anomalies: one in which one or more (or all) of thevoltage sensors either go down or provide unreliable data; and anotherin which an RRH 120 was configured or reconfigured incorrectly in thefield. In the latter case, controller can maintain proper operation ofPSU 130 providing power to RRH 120. These anomalies are identified andmitigated in the process described below.

FIG. 5 illustrates an exemplary process 500 for robust operation ofsystem 200 via the functions of controller 220.

In step 510, processor 222 receives voltage data streams from each ofthe voltage sensors 210 in a manner substantially similar to steps 310,330, and 350 of process 300.

In step 515 processor 222 executes instructions to validate the incomingvoltage samples. In doing so, processor 222 may buffer the voltagesamples from each voltage sensor 210 and compare them with a respectiveactive voltage range. The active voltage range may be a range ofvoltages that would be consistent with proper operation of the RRH 120as well as proper function of voltage sensor 210. If the voltage samplelies within that range, process 500 proceeds via decision gate 515 tostep 520, in which processor 222 stores the voltage sample and its timestamp in non-volatile memory 223.

Following process 500 onward through steps 520-555, processor 222performs functions substantially similar to steps 420-450 of process 400in FIG. 4.

Returning to decision gate 515, in the event that the voltage samplelies outside the active voltage range, then process 500 proceeds to step535, in which processor 222 executes instructions log error data innon-volatile memory 223. Error data may include the voltage valuereceived in step 505 along with time stamp, the identity of the PSU 130and RRH 120.

In step 540, processor 222 executes instructions to determine whetherany of the voltage samples corresponding to other RRH/PSU pairs are alsooutside their respective active voltage ranges. In doing so, step 540may include multiple iterations of steps 505 and 510 for the othervoltage sensors 210. Alternatively, step 540 may be omitted and thestatus of the other voltages will be subsequently determined whencontroller 220 executes process 500 for the other PSU/RRH pairs.

In step 545, processor 222 executes instructions to issue an alert toexternal network 260 via third bus interface 228.

In step 547, processor 222 executes instructions to retrieve thepreviously commanded voltage (as done in a previous iteration of process500) and sets this value as the commanded voltage to be issued to thePSU 130. In this case, controller 222 prevents an anomalous voltagereading from causing an instability or an improper voltage command tothe PSU 130.

In a variation of process 500, validation step 515 and subsequent steps535-547 may be incorporated into step 520. In this case, this modifiedversion of process 500 may appear similar to process 400, but with therun pre-filtering model step 420 incorporating the validation andpossible voltage replacement with the previous voltage as describedabove with reference to steps 515 and 535-547. However, in the eventthat the voltage sample lies outside the active voltage range, then theprocess may bypass the pre-filtering step described above and simplyreplace the new command voltage with the previous command voltage.

FIG. 6 illustrates another embodiment of the disclosed system 600.Exemplary system 600 includes one or more PSUs 630, each of whichprovides power via a respective power cable 140 to a corresponding RRH120. System 600 also includes a controller 220. All of these componentsmay be substantially similar to the like-numbered components in FIG. 2a. System 600 also includes a sensor 610 coupled to each PSU 630 and thecontroller 220. Further illustrated are parasitic capacitances 620, eachof which may be individually characteristic of its respective RRH 120.Please note that parasitic capacitances 620 are inherent to the givenRRH 120, and are not separate dedicated capacitors specifically coupledto the RRH 120 at the connection to power cable 140.

Sensor 610 is a current sensing device that measures the current inpower cable 140. It may also sense the voltage output at PSU 630.Controller 220 may communicate with each sensor 610 via a sensor databus 635. Sensor data bus 635 may be implemented according to any knowninstrumentation serial bus standards, or alternatively via an Ethernetconnection. Preferably, each sensor 610 should be located in closeproximity to its corresponding PSU 630. Alternatively, sensor 610 may beintegrated into PSU 630.

Controller 220 communicates to each PSU 630 via PSU data bus 235, in amanner substantially similar to system 200 in FIGS. 2a and 2b . In afurther variation, PSU data bus 235 and sensor data bus 635 may be asingle serial bus. It will be readily understood that such variationsare possible and within the scope of the disclosure.

PSU 630 may differ from PSU 130 of FIG. 2a in that PSU 630 is configuredto operate in a mode in which it can output a signal of a singlefrequency at a time, whereby the frequency can be controlled so that PSU630 can output a single frequency signal for a specific duration, andthen do the same at a different frequency. In doing this in a sequence,PSU 630 can transmit a sequence of single frequency pulses at, forexample, incrementally increasing frequencies from a minimum frequencyto a maximum frequency (a successive frequency pulse sequence), in analternative example, sensor 610 may be have an additional subsystem thatprovides this successive frequency pulse sequence, instead of PSU 630.

FIG. 7 illustrates an exemplary process 700 by which system 600characterizes each combination of power cable 140 and parasiticcapacitance 620. With this complete, controller 220 can command each PSU630 to output a specific voltage that will result in a desired voltageat the remote connection between power cable 140 and RRH 120. Controller220 may execute process 700 on startup, as well as at other times afterany maintenance or servicing in which one or more RRHs 120 are powereddown. Similar to process 400, processor 222 may execute process 700 onceper individual PSU 630, thus in one iteration per PSU 630.Alternatively, processor 222 may execute each step of process 700 foreach and all PSUs 630 before proceeding to the next step. It will bereadily apparent that such variations to process 700 are possible andwithin the scope of the disclosure.

In step 710, controller 220 sets each PSU 630 to output an AC voltage ata predefined frequency. This is the first frequency of the successivefrequency pulse sequence, each of which may be stored in non-volatilememory 223 and retrieved by processor 222. Controller 220 sends acommand to each PSU 630 to output a pulse of specific duration at thisfrequency. For example, controller 220 may command each PSU 630 to issuea sequence 10 pulses per frequency, from frequencies ranging from 1 Hzto 10 MHz on a logarithmic scale, and at an amplitude of substantially 5Vpp. The pulse duration would vary with frequency.

In step 720, PSU 630 transmits a pulse at the set frequency. Sensor 610measures the current and voltage of the output of PSU 630 and continuesto monitor power cable 140 for a returned pulse.

In step 730 and 740, if sensor 610 detects a returned (reflected) pulse,it measures the time between the transmission of the pulse and thearrival of the reflected pulse, and the amplitude of the reflectedpulse. Sensor 610 then transmits the return time and the measuredamplitudes to controller 220, whereby processor 222 stores the data innon-volatile memory 223.

If there is another frequency to transmit, processor 222 executesinstructions to increment the frequency, or otherwise step to the nextfrequency, and repeat steps 720-740. Once steps 720-740 have beenexecuted for all of the frequencies, processor 222 executes instructionsto identify the frequency that yielded the highest amplitude return, andretrieve the return time corresponding to that frequency. With thisinformation, in step 770, processor 222 may calculate the length of thetransmission path through power cable 140. Once the length of the powercable 140 is known, then given the gauge of the power cable 140 (whichis also known), in step 780 processor 222 then calculates the totalresistance of the transmission path. Processor 222 then calculates therequired voltage offset that controller 220 must command PSU 630 tooutput in order to provide the desired voltage at the end of power cable140 at the RRH 120. Processor 222 may then store the resistance and thevoltage offset corresponding to the specific PSU/cable/RRH combination.

With this done, system 600 may execute appropriately modified versionsof processes 400 and/or 500.

In another exemplary embodiment, system 600 may have a variation inwhich the power cables 140 are characterized by generating a set of VIfactors for each cable, as opposed to testing at different frequencies.In this case, each sensor 610 may be an inductive current sensor, whichmeasures the current in power cable 140.

This embodiment may include a test mode, in which controller 220 setsdifferent command voltages for each PSU 630. For each voltage setting,sensor 610 measures the current through each respective power cable 140and a temporary voltage sensor (not shown) measures the voltage at theinput to each RRH 120. In this test mode, controller 220 cycles througha series of voltages and records the commanded voltages, sensedcurrents, and sensed RRH voltages in non-volatile memory 223. In doingso, controller 220 has established a set of VI factors for operationalcontrol of the voltage at each RRH.

In nominal operation of system 600 according to this embodiment,controller 220 sets the voltage at each PSU 630 to the voltagecorresponding to the desired voltage at each RRH 120 and monitors thecurrent in each power cable 140 via respective sensor 610. Depending onthe sensed current, processor 222 in controller 220 may adjust thecommanded voltage so that the combination of commanded voltage andsensed current corresponds to the proper voltage at RRH 120. If powerdemand increases in RRH 120, PSU 630 may compensate in order to keep thecommanded voltage constant. In this case, processor 222 may increase thevoltage to be set to PSU 120 in order to compensate for the increase inpower demand by increasing the voltage and keeping the currentsubstantially constant. In doing so, the voltage at RRH 120 ismaintained, and the current in power cable 140 is also controlled sothat the loss in power cable 140 does not increase.

It will be understood that variations to system 200 are possible andwithin the scope of the disclosure. For example, although three pairs ofPSUs 130 and RRHs 120 are illustrated, any reasonable number of RRHs 120and PSUs 130 may be employed. Further, a single PSU 130 may providepower to more than one RRH 120. Also, data bus 230 may be integratedinto power cable 240 such that it is a hybrid cable that provides bothpower and data connectivity between a PSU 130 and its correspondingRRH(s) 120. Similarly, sensor data bus 230 and PSU data bus 235 maycomprise a single serial bus, in which case first and second businterfaces 224 and 226 may be a single interface bus.

What is claimed is:
 1. A system for controlling power to a cell tower,comprising: a plurality of tower top equipment units, each of the towertop equipment units having a remote radio head and a correspondingvoltage sensor; a plurality of power supply units; and a controllercoupled to the voltage sensors and a purality of power supply units,wherein the controller is configured to receive a plurality of voltagesamples from each of a plurality of voltage sensors, the voltage samplescorresponding to power provided by each of the power supply units to acorresponding remote radio head, pre-filter each voltage sample, andissue a voltage command to each of the power supply units, wherein eachvoltage command corresponds to a pre-filtered voltage sample wherein thecontroller is configured to perform a discovery scan between each of thethe power supply units and the tower top equipment units, the discoveryscan comprising the steps of: commanding a first power supply unit togenerate a first test voltage; receiving a first plurality of voltagesamples from the plurality of voltage sensors; commanding the firstpower supply unit to generate a second test voltage; receiving a secondplurality of voltage samples from the plurality of voltage sensors;identifying a first tower top equipment unit by identifying a responsivevoltage sensor that responds to the first test voltage in the firstplurality of voltage samples, and responds to the second test voltage inthe second plurality of voltage samples; and storing an identifiercorresponding to the first power supply unit and an identifiercorresponding to a first remote radio head.
 2. The system of claim 1,wherein the first test voltage comprises a maximum voltage, and thesecond test voltage comprises a minimum voltage.
 3. The system of claim1 wherein the identifier corresponding to the first power supply unitcomprises a bus address.
 4. The system of claim 1 wherein the discoveryscan further comprises: commanding a second power supply unit togenerate a maximum voltage; receiving a third plurality of voltagesamples from the plurality of voltage sensors; commanding the secondpower supply unit to generate a minimum voltage; receiving a fourthplurality of voltage samples from the plurality of voltage sensors;identifying a second tower top equipment by identifying a responsivevoltage sensor that responds to the maximum voltage in the thirdplurality of voltage samples, and responds to the minimum voltage in thefourth plurality of voltage samples; and storing an identifiercorrsponding to the second power supply unit and an identifiercorresponding to a second remote radio head.
 5. The system of claim 1,wherein the discovery scan further comprises the steps of filtering ofeach voltage data sample which further comprises the steps of: comparingan incoming voltage sample with an active voltage range; if the incomingvoltage sample is within the active voltage range, pre-filtering theincoming voltage sample, comparing the pre-filtered incoming voltagesample with a voltage setpoint, and storing the pre-filtered incomingvoltage sample; and if the incoming voltage sample is outside the activevoltage range, issuing an alert, and retrieving a previous pre-filteredvoltage sample and storing the previous pre-filtered voltage sample asthe pre-filtered incoming voltage sample.
 6. The system of claim 5,further comprising a plurality of power supply units coupled to arespective plurality of remote radio heads by a plurality of powercables, configured to perform the steps of: a) setting the first powersupply unit to a first frequency; b) sending an output pulse at thefirst frequency; c) measuring a return time for a return pulsecorresponding to the output pulse; d) measuring an amplitude of thereturn pulse; e) repeating steps a) d) for a plurality of frequencies;f) identifying a matching frequency corresponding to the highest amplitude return pulse; g) calculating a power cable length based on the returntime corresponding to the matching frequency; h) calculating a powercable resistance based on the power cable length; and i) calculating anoffset voltage based on the power cable resistance.
 7. The system ofclaim 6, wherein the plurality of frequencies comprises a range from 1Hz to 10 MHz in a logarithmic scale.
 8. The system of claim 1, whereineach of the power supply units is coupled to a respective tower topequipment unit by a power cable, and further comprises: an inductivelycoupled sensor coupled to its respective power cable; the controllercoupled to the plurality of power supply units and its correspondinginductively coupled sensor, the controller having a procesor and anon-volatile machine readable memory encoded with instructions foroperating in a test mode and an operation mode, wherein the test modeincludes setting a series of voltages at each of the plurality of powersupply units, measuring a resulting current with a correspondinginductively coupled sensor and calculating a VI factor corresponding toeach voltage and the resulting current, and wherein the operation modeincludes setting each of the power supply units to a control voltage inresponse to a sensed current, wherein the control voltage is a functionof the sensed current and a respective VI factor.
 9. The system of claim8, wherein the inductively coupled sensor comprises an inductive currentsensor.
 10. The system of claim 6, wherein the output pulse comprises anamplitude of 5Vpp.
 11. The system of claim 9, wherein the inductivelycoupled sensor comprises an inductive voltage sensor.
 12. The system ofclaim 11, wherein the inductively coupled sensor comprises the inductivecurrent sensor and an inductive voltage sensor.
 13. The system of claim12, wherein the inductively coupled sensor is coupled to its respectivepower cable at a top of the power cable, proximate to the tower topequipment.
 14. The system of claim 13, wherein the inductively coupledsensor is coupled to its respective power cable proximate to at leastone of the plurality of power supply units.